This invention relates to an electron emitting element of the field emission type, and more particularly to an electron emitting element suitable for use as an electron source for various kinds of display devices, a light source, an amplifier element, a high-speed switching element, a sensor or the like.
A vertical-type electron emitting element which is typical one of an electron emitting element of the field emission type is generally constructed in such a manner as shown in FIG. 11. More specifically, it includes a substrate 100 doped with impurities in high concentration, resulting in being provided with high conductivity. On the substrate 100 is arranged an insulating layer 101 made of SiO.sub.2, which is formed therein with cavities 102. In each of the cavities 102 is arranged an emitter 103 made of molybdenum (Mo) so as to serve as an electron emitting section. Also, the electron emitting element includes a Mo thin film deposited on the insulating layer 101 in a manner to surround the emitter 103 so as to function as a gate electrode 104.
In the electron emitting element constructed as described above, when the gate electrode 104 is biased within the range of tens to hundreds V against the substrate 100, an electrical field as high as 10.sup.6 to 10.sup.7 V/cm is caused to be produced between the distal end of the emitter 103 and the gate electrode 104, so that electrons of hundreds mA in all may be emitted from the distal end of the emitter 103.
FIG. 12 shows a conventional display device in which the so-constructed electron emitting element is used as an electron source. The conventional display device is disclosed in, for example, Japanese Patent Application Laid-Open Publication No. 221783/1986.
The conventional display device is constructed in such a manner that a plurality of conductive films 112 are arranged on an insulating substrate 110 so as to extend in the direction of columns 111. On the conductive film 112 are provided cone-like emitters 113 of the field emission type and an insulating layer 114. On the insulating layers 114 are arranged a plurality of grids 116 in a manner to extend in the direction of rows 115. The grids 116 each are formed at the portion thereof opposite to each of the cone-like emitters 113 with an aperture or hole.
The display device also includes a transparent substrate 117. On the surface of the transparent substrate 117 opposite to the insulating substrate 110 are deposited a transparent conductive film 118 and a phosphor layer 119 in a manner to be laminated in order. The conductive film 118 and phosphor layer are arranged all over the substrate 117. The insulating substrate 110 and transparent substrate 117 cooperate with side plates (not shown) to form an envelope, which is then evacuated to a high vacuum.
Now, the manner of operation of the conventional display device constructed as described above will be described hereinafter.
A positive potential is constantly applied to the transparent conductive film 118. In response to a display signal, a predetermined potential difference is applied between the conductive film 112 of each of the rows 115 and the grid 116 of each of the columns. This causes a suitable electric field to be formed between the grid 116 to which the potential difference is applied and the cone like emitter 113, resulting in electrons being emitted from the pointed distal end of the emitter 116. The so-emitted electrons travel through the hole of the grid 116 and then impinge on the phosphor layer 119, leading to light-emission or luminance of the phosphor layer 119.
Thus, an image is displayed depending upon the display signal.
FIG. 13 shows a horizontal-type electron emitting element which is another one of an electron emitting element of the field emission type, which is disclosed in, for example, in Japanese Patent Application Laid-Open Publication No. 33833/1989.
The horizontal-type electron emitting element includes an insulating substrate 200, on which an emitter 202 provided at the central portion thereof with a triangle projection 201 is arranged. Also, the substrate 200 is provided thereon with a gate 204 in a manner to be adjacent to the emitter 202. The gate 204 is formed with an aperture or hole 203 at the portion thereof corresponding to the projection 201. Also, the electron emitting element includes a secondary electron-emitting electrode 205 in a manner to interpose the gate 204 between the emitter 202 and the electrode 205 and be in parallel to the gate 204.
In the horizontal-type electron emitting element constructed as described above, when predetermined potential differences are applied between the emitter 202 and the gate 204 and between the gate 204 and the secondary electron emitting electrode 205, respectively, electrons emitted from the pointed distal end of the emitter 202 impinge through the aperture 203 of the gate 204 onto the secondary electron-emitting electrode 205, so that secondary electrons are emitted from the secondary electrode 205.
Unfortunately, when the conventional electron emitting element of each type described above is driven in an airtightly or hermetically sealed envelope or at a vacuum atmosphere as low as 10.sup.-6 to 10.sup.-7 Torr, it causes some disadvantages. More specifically, mounting of the electron emitting element in an airtight envelope causes the emitter to be polluted during the mounting operation, so that the emitter may be significantly increased in emission threshold. Also, driving of the electron emitting element in a low vacuum atmosphere causes the emitter to absorb any gas in the atmosphere, resulting in the electron emitting element being increased in work function in a short period of time. This leads to disadvantages of causing an emission efficiency of the element to be reduced and/or its emission threshold to be increased. The disadvantages are remarkably exhibited particularly when the electron emitting element of the field emission type is used while being mounted in the airtight envelope. Use of a filed emission (FE) cathode in the form of being mounted in a hermetic envelope is never realized unless the disadvantages are solved.